The invention has arisen specifically relative to so-called Insituform (Registered Trade Mark) tube used to proprietary process or method of "no-dig" relining of typically underground services or utilities supply pipes. A flexible tube of coated woven or felt-type sheet material having a sewn seam is readily installed into an existing said pipe, say by inversion using a head of water; and usually before being impregnated with thermoset plastic material, such as polyester or epoxy resin. Water pressure within such tube of appropriate size can expand and force it into tight fit against interior of the host pipe; and subsequent heating of such water, typically as circulated from an above-ground boiler, can aid curing of the resin to form a solid durable lining or re-lining of existing host pipe, typically of diameters from about 100 millimetres or less to about 3 metres or more.
Typical such Insituform tube is made on a production line by sewing together long edges of folded-over plastics coated felt material strip of appropriate width to form a "lay-flat" tube of desired size. Coating on the felt material is usually applied by the felt manufacturer/supplier before shipping to the Insituform tube production plant, and is typically of thermoplastics material, specifically polyurethane (PU) or polyethylene (PE), in a thickness usually up to about 0.5 millimetre. It is important that Insituform tube affords a fully sealed pipe lining after installation; and any incidence of leaks or dripping of impregnating resin during installation are problematic, i.e. need to be avoided. Sealing of said seams of lay-flat tube as supplied to field sites for installation is thus required, and needs to be of a good and reliable nature. Two methods seam sealing are known or have been proposed.
One such method relies on solvent bonding a plastics tape over the seam, typically about 65 millimetre width of about 0.3 millimetre thick PU tape that is bonded using tetrahydrafuran (THF) solvent. Whilst equipment required is readily incorporated into production lines for lay-flat sewn-seam tube that is PU-coated, practical solvents that afford similar possibilities for PE-coated felt are not available. Moreover, even for PU-coated tube, use of THF can give rise to health and safety concerns.
The other such method relies on extruding a bead of molten plastics onto said sewn seams, then rolling still-soft extruded bead flat over and to cover/seal the seam, including fusing with the coating. Whilst this method is applicable in principle to either PU or PE coatings, there are practical problems. Thus bead extrusion is not readily started and stopped at will due to heating requirements and control relative to flow characteristics that must be achieved and maintained, which conflicts with norms for sewn-seam Insituform tube production lines where sewing machine and felt guidance operations tend to be subject to frequent interruptions, thus stops. Indeed, two separate production stages would appear to be required, with inevitable result of duplication of materials stacking and feed systems and extra staff requirement. Also, bead processing speed is limited by cooling rate of the plastics material concerned, which tends to be particularly slow for PE, thus further leads to requirement for substantial length of run-off space etc before further handling and/or use of cooling equipment which would be expensive to install. Extrusion thus looks an expensive option in terms of sophisticated capital equipment for extrusion possibly also cooling.